Mobile radio communication between individual vehicles, as well as base stations, has a limited range for direct communication. To extend the range of communication, transmitting and receiving stations, repeaters, are provided which have antennas mounted on towers, tall buildings, mountains, etc., so that each covers an extended geographic area. By properly spacing the towers, communication can be maintained between vehicles and base stations over a very large area. To even further extend the range of operation, the stations can be interconnected with wired or wireless networks.
In typical operation with repeater stations, a mobile radio in a vehicle transmits a request to a particular station for access to any one of a plurality of communication channels available at that station. If the one requested station hears the request from the mobile radio, and a channel is available, that one station allocates a channel which is then used by the mobile radio for two-way communication. However, to insure continuity of communication over a large geographic area, the zones of the transmit/receive stations must overlap to some extent. Further, variations in propagation conditions can change the distances at which a mobile radio can communicate with a station. Thus, it frequently occurs that a request for a channel from a mobile radio is not serviced by the requested station. When this occurs, the mobile radio sequences to a second station and addresses its service request to that station, even though the second station may be further away. If the second addressed station receives the request and allocates a channel, that station must also use a narrow-band communication channel back to the original station and further request a communication channel for use at that station. In the instances where the mobile radio requires communication only with other mobile radios in the zone of the first station, there is an unneeded allocation of the channel at the second station together with an unneeded allocation of an inter-station narrow band communication link. Thus, there is a waste of channel capacity and an inter-zone link when communication is only required within one zone.
The radio frequency spectrum available for two-way communication is very limited, and is therefore very valuable. Thus, any excess allocation of channels or links in the interconnection network is a substantial economic loss.
In view of this problem, there exists a need for a method and apparatus for limiting the allocation of communication channels in a two-way radio system to the absolute minimum number of channels and links required for the desired communication.